It is well-known that X-ray scanners are used in medicine to obtain information about the interior of a patient for the purpose of diagnosis and/or therapy. In the process, the different attenuation properties of the various substances with respect to X-ray radiation generated by the X-ray scanner are utilized for providing contrast in the image. Compared to soft tissue, bone tissue has very different attenuation properties. As a result of the high image contrast connected therewith, bone structures can be analyzed in a simple fashion. However, vessels and organs, which do not differ substantially in terms of their attenuation properties from the surrounding soft tissue, cannot be examined in a conventional fashion as a result of the lack of contrast.
However, it is precisely such examinations that have become very important in diagnostic medicine. Angiographic examinations are, in the meantime, used to diagnose a number of different vessel disorders. Thus, for example, angiographic examinations can be used to diagnose arterial disorders such as e.g. arteriosclerosis and vessel narrowing connected therewith, sacculation and impending vascular occlusion, and also venous disorders, such as e.g. thromboses.
In radiology, C-arm systems have up until now been predominantly used for angiographic examinations. The flat-panel detectors available in these systems can generate projection images with high spatial resolution and high coverage of the examination region. As a result of the small differences in the attenuation properties between the vessels and the surrounding soft tissue with respect to X-ray radiation, angiographic examinations are carried out using a contrast agent in order to increase the image contrast. Vessel structures and organs connected thereto are made visible by indirect means in the projection image as a result of the volume taken up by the contrast agent. Depending on the medical question, it may be desirable in the process to generate a projection image which merely contains the contrast agent. That is to say image structures of bone and soft tissue should be suppressed in this case.
A conventional technique for masking such image structures is provided by digital subtraction angiography. In the process, a native projection image of the examination region without contrast agent is firstly recorded from a previously set projection direction by means of a C-arm system. In a second step, a series of projection images are acquired from the same projection direction after the contrast agent was injected. The previously acquired native projection image is subsequently subtracted from these projection images, and so only differences between the data become visible. Thus, ideally, it is merely the contrast-agent filled vessel and organs that can be seen in the resulting projection image while the bones and the soft tissue are eliminated from the image.
A significant disadvantage of this method emerges from the fact that the projection images used for the subtraction were recorded at different times. A patient movement that took place between the recording times, as already caused by breathing, must be compensated for by registering the two projection images before the actual subtraction. Sufficiently good results can in general only be achieved in the process if the registration is carried out on the basis of computationally expensive affine image transformations.
However, despite image registration, many situations do not allow mathematically completely accurate estimates of the patient movement, which leads to the formation of a certain amount of shadowing in the region of erroneously registered image structures in the resulting projection image.
These days, computed tomography systems are also used for examining vessels. The so-called CT angiography, also abbreviated as CTA, is based on a 3D image reconstruction, in which projections are acquired from a multiplicity of different projection directions during a rotation of the recording system around the patient and combined with each other to form a slice image. The vessels to be illustrated in the slice image are subsequently segmented from the slice images using image-processing algorithms and output in freely rotatable 3D representations. Even without generating a native slice image, the CTA examinations are disadvantageous in that the patient is exposed to an increased X-ray dose compared to an angiographic examination using a C-arm system as a result of the multiplicity of projections from different angular directions required for the image reconstruction.